1. Field of the Invention
This invention relates to a method of removing acid gas pollutants from a gas stream and in particular a method of removing sulfur oxides and/or nitrogen oxides from gas streams.
2. Description of the Prior Art
It has long been recognized that the presence of noxious gases, for example, sulfur dioxide, sulfur trioxide, nitrogen oxide and nitrogen dioxide in gas streams that are released to the atmosphere causes acid rain and is extremely damaging to the environment. Therefore, numerous methods and processes have been devised to remove these gas pollutants from a gas stream prior to releasing said stream into the environment. Also, as these gas pollutants are usually a product of combustion, one known method of controlling the sulfur content of gas streams is to use low sulfur content fuels. In addition, processes have been devised to pretreat the fuel to remove the sulfur before combustion. The supplies of low sulfur fuel are rapidly becoming depleted, necessitating the use of higher sulfur content fuels in the future.
One known method of controlling the emission of sulfur dioxide and other sulfur gases is to add a reagent, for example, limestone or dolomite directly into the combustion chamber where it reacts with the sulfur during combustion and the sulfur so removed becomes a constituent of the ash. Another known method of removing gas pollutants is to treat the gaseous products of combustion by reaction with a dry reagent (i.e. dry scrubbing) or by contacting the flue gases with a liquid or liquid slurry reagent to absorb the sulfur dioxide, sulfur trioxide and nitrogen oxide components (i.e. wet scrubbing). Known wet scrubbing reagents are aqueous solutions of various chemicals such as calcium carbonate, calcium hydroxide, magnesium oxide, magnesium carbonate, sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonium sulfites, etc. There is a great deal of literature available describing these wet scrubbing reagent and methods of using them. However, all of these known reagents suffer from one or more disadvantages in that they do not remove sufficient gas pollutants from the gas stream; or, their use is extremely expensive; or, they create further pollutants that must be further treated; or, they can be dangerous to use; or, they require the use of highly-skilled operators. It is also known to use coal to control sulfur oxides or nitrogen oxides. However, the coal is not used in a wet scrubber but is used as an adsorbent where the sulfur oxides and nitrogen oxides are adsorbed directly on a char surface of the coal and become part of the char. No ion exchange is involved in this prior method.